NASA Technical Reports Server (NTRS) 19930016965: Advanced technology development multi-color holography

39 Pages·1.8 MB·English

by  NASA Technical Reports Server (NTRS)

ADVANCED TECHNOLOGY DEVELOPMENT
MULTI-COLOR HOLOGRAPHY
(Final Report)
Contract No. NASS-38609/D.O. 30
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Prepared by
Chandra S. Vikram
Center for Applied Optics, The University of Alabama in Huntsville
Huntsville, Alabama 35899
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Prepared for
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National Aeronautics and Space Administration
George C. Marshall Space Flight Center
Marshall Space Flight Center, Alabama 35812
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(NASA-CR-192451) ADVANCED N93-26156
TECHNOLOGY DEVELOPMENT MULTI-COLOR
HOLOGRAPHY Final Report, 26 Mar.
1992 - 24 Jan. 1993 (Alabama Unclas
Univ.) 39 p
G3/35 0163071
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January 1993
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CONTENTS
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1. INTRODUCTION ............................................... 2
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AVAILABLE TECHNIQUES FOR PRECISE FRINGE LOCATION
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MEASUREMENT .............................................. 3
2.1. Heterodyne interferometry ..................................... 3
2.2. Quasi-heterodyne interferometry ................................. 4
2.3. Phase-shifting interferometry .................................... 5
2.4. Effective technique for multi-color holography ....................... 7
EXPERIMENTATION ........................................... 7
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3.1. System-description ............................................
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3.2. Rotating plate phase shifter ..................................... 10
3.3. Calibration of a phase shifter ................................... 12
3.4. Experimentation with sugar solution .............................. 15
EXTENSION TO MORE THAN TWO WAVELENGTHS ................ 16
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4.1. Advantages of using more than two colors ........................ 16
4.2. Disadvantages ............................................... 17
4.3. Special beam intensity ratio needs in multi-color holography ............ 17
N 4.4. Summary on using more than two wavelengths ....................... 22
OTHER ANALYSIS METHODS ................................... 23
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5.1. Deflectometry ............................................... 23
5.2. Speckle techniques ........................................... 25
5.3. Confocal optical signal processing ................................ 27
5.4. Video holography ............................................ 28
5.5. Phase shifting technique related applications ........................ 28
BREADBOARD DESIGN ........................................ 29
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6.1. Optical fibers ............................................... 29
6.2. Laser sources ............................................... 30
6.3. Test cell ................................................... 30
7. CONCLUSIONS ................................................ 31
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Appendk-A ..................................................... 32
References ...................................................... 33
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1 INTRODUCTION
One of the important techniques developed by NASA-George C. Marshall Space Flight
Center for the materials processing in space program involves holography. The Fluid
Experiment System (FES) employs holography to monitor heat and mass transfer in the
crystal growth cell. NASA KC-135 aircraft flying in a parabolic trajectory, Spacelab III
Mission as well as more recent First International Microgravity Laboratory (IML) employed
holography as one of the diagnostics tools in a reduced gravity environment.
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There are two main advantages of using holography. First, it does not require extremely high
quality optical components. Phase errors introduced during the recording can be eliminated
during the reconstruction. The other advantage is strategic for the space program.
Holography stores and reconstructs the optical wavefront. This wavefront can be used for
any analysis application not necessarily standard holographic interferometry. The standard
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storage unit on the flight or the hoIocamera actually replaces several systems simultaneously.
In fact, part of this report identifies some possible uses of the reconstructed wave front other than
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the standard holography and holographic interferometry.
Holographic interferometr), with one wavelength provides valuable information about the
m refractive index variations in the fluid cell. As such, temperature and concentration effects
can not be separated from the index information. Use of thermocouples to measure
temperature is intrusive and possible at a limited number of points in the test section. Thus,
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u for temperature and/or concentration analysis, the conventional one-color interferometry
has a restricted capability.
w On the other hand if two holograms at different wavelengths are available, then temperature
and concentration effects can be separated. Based on the proposal of Ecker et al.,
experimentation followed at NASA-Marshall Space Flight Center (laboratory as well as KC-
135 aircraft). Success of these historical developments __(basically dealing with succinonitrile
= : based transparent systems) resulted in renewed NASA enthusiasm in two-color holography.
A more critical study of the process' and related publications _'° followed. These critical
studies as well as another publication _ clearly indicated the need of very accurate fringe
counting procedure in two-color interferometry particularly in most conservative situations
where very small temperature and concentration changes are to be encountered. Otherwise,
two interferograms from two wavelengths may not be linearly independent in the practical
sense.
In this connection, this report deals with several strategic aspects of two- (or multi-) color
holography. These are:
• To review available techniques for precise fringe location measurement of phase media.
This study is needed for proper fringe measurement technique selection. The important
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aspects are the sensitivity needs as well as the suitability for the NASA-FES environment.
• Experimentation to validate the predicted accuracy of the fringe counting procedure. The
goal is to find the maximum obtainable practical fringe position accuracy in the case of fluids
and in a typical NASA-FES environment.
• Extension to multi-color holography. This section deals with the advantages and
disadvantages of using more than two wavelengths. Accuracy for temperature and
concentration measurements, new system hardware needs, and possible lasers will also be
considered.
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• Based on the above studies, a design of the experimental prototype of multi-color
holography will be described. Again, typical temperature and concentration gradients in
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solution crystal growth and other NASA-FES related experimental situation are specifically
considered.
• Other analysis methods. As stated earlier, holography reconstructs the wavefront.
Traditionally, holographic methods such as holographic interferometry are used. However
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the wavefront can in principle be used in the non-traditional ways. Some of these possible
approaches and their relative merits will be considered in this section.
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2 AVAILABLE TECHNIQUES FOR PRECISE FRINGE LOCATION MEASUREMENT
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IJ Visual or routine quantitative methods of fringe position measurements have the capability
of 1/5 - 1/10 fringe order. '2As stated in Section 1, this may not be enough for certain
aspects of quantitative multi-color interferometry. Therefore, this section deals with basic
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R techniques available for accurate fringe measurement and their application to fluids. It is
worth mentioning here that the termfluid is often used forgases in literature. For liquids the
U precise fringe measurement techniques have been rarely used.
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2.1 Heterodyne interferometry
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This kind of interferometry involves two interfering waves with different frequencies. Two
waves of complex amplitudes A, exp(2_'f, t) and A_ exp[i(27rf,t + _0)] at the time t are
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allowed to interfere. A,, A, are the absolute amplitudes; f,, f, are the frequencies; and qo
is the phase difference. The photodetector output is:
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]A, exp(27rif,t) + A_ exp[i(2-n'f_ t + _p)]l _
N = A_+ A2_+ 2At A2cos[2"rr(f2-f_)t + _].
f,- f, is typically in MHz range. The phase term _ooriginally carried at 10 _'Hz in the optical
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domain is now carried by the sinusoidal electric signal in the range of 106Hz.
In this kind of interferometry, there is no visual fringe pattern. However, at a given time,
two photodetectors (one serving as a reference) at different locations can be used. The
phase difference between the electrical outputs serves to generate a map of tO.Generally,
one fixed detector serves as the reference and the other scanning detector serves to generate
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the phase map over the cross section. As described by Sirohi and Kothiyal, the desired
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frequency shifts can be provided by rotating polarization components, moving diffraction
gratings, acoustooptic Bragg cell, or using a laser with two frequency outputs. So far,
heterodyne interferometry has been used in optical testing, profilometry, small displacement
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and vibration analysis, etc.
Farrell, Springer and Vest" applied heterodyne holographic interferometry to study
temperature and concentration in gas mixtures. The scene was natural convection boundary
layers in air adjacent to a heated surface. Basically, the refractive index was measured with
independent temperature determination using fine-wire thermocouples. The two reference
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beam method, t3where one beam's frequency is altered using an acoustooptic modulation,
was used.
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Heterodyne interferometry offers high spatial resolution and 1/1000 fringe order
measurement capability.'6 However, the method involves point-by-point analysis, requires
special equipment such as an acoustooptical modulator and phasemeter. Besides the
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hardware, the operational aspects are complex and processing times are long. Consequently,
the method is now not common in research efforts.'7
2.2 Quasi-heterodyne interferometry
The word quasi-heterodyne is often used for phase-shifting (details in Section 2.3)
interferometry.'6"' In the present study by quasi-heterodyne we mean very specialized phase
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shifting procedures different from simple two-beam interference with phase shifting and
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digital data processing. In that sense, the quasi-heterodyne procedures were the attempts to
simplify the heterodyne procedures before the actualphase-shifting procedures settled down
and were well understood.
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= : In one application, _8the optical path difference between the interfering wavefronts is varied
linearly with time. The irradiance output at different times can yield different intensities.
The desired phase r_ can be evaluated from these intensities. The method requires strictly
linear optical path variations, fast electronics for the data collection, and phase corrections
of the computed values. Nevertheless, the method can yield the spatial phase map without
the scanning detector.
There are several articles dealing with two reference beams'"t6'_'_ in flow research. In this
double exposure technique, two reference beams are used - one for each exposure. During
reconstruction, the mutual phase between the reconstruction beams is altered to generate
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different interference patterns. The irradiances of these patterns can be used to solve for
the unknown phase map over the cross-section. These phase shifting/digital techniques
included several examples in flow research:
• Plume from a heated wire in a cross flow. 12
z : • Helium jet injected in still air. 12
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• Laboratory simulated tornado. 19
w • Density distribution in an axisymmetric supersonic jet of air. _
Reference 20 provides a good summary of the digital/phase shifting procedure in flow
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These two reference beam methods definitely are valuable in obtaining the phase map over
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the entire cross-section by fringe shifting and digital data analysis. Dealing with the NASA-
FES system, significant hardware changes are necessary with these methods.
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A more recent dual reference approach "1utilizes two reference beams for the recording but
only one for the reconstruction. The desired phase shifts are obtained by changing the
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viewing directions.
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2.3 Phase-shifting interferometry
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Phase shifting interferometry, also called digital interferometry, is suitable for rapid
gIw_ measurements of whole field phase distribution. Although the customary practical limit of
the measurable optical path change has been _ _./200, it is approaching that obtainable by
the heterodyne technique. In the recent publication by Lai and Yatagai, _'the limit is _./500!
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Here k is the wavelength of the light used. In the work due to Schwider, _ computer
simulations show the accuracy of k/lO00 when an error function is subtracted from the
measured phase values.
Let us introduce the basic concept of the phase shifting procedure. The intensity at a point
(x,y) in the interference pattern can be written as
(1)
I, = A(x,y) + B(x,y)cos[c(x,y) + eL],
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where A(x,y) is the average intensity, B(x,y) relates to the fringe contrast, and _(x,y) is the
phase of the wavefront to be measured, a, is a known applied phase shift in the j'_ set of the
frame of data. The basic procedure is that with known a, _(x,y) can be evaluated from I_
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(x,y) values. The intensity values can easily be stored in data frames and the quantitative
values/plots of _x,y) can easily be obtained by a computer.
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Let us discuss one of the specific procedures of phase shifting interferometry. Suppose t_,,
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a2 and a3 are 0, 2"rr/3 and -2"n'/3 respectively, then eq.(1) yields
tan qo= 3'_(I, - I,)/(2I,- I2- I3). (2)
Notice that local visibility and background terms A and B are completely eliminated in this
evaluation.
The phase shifting procedure is general to interferometry. For holographic interferometry,
the phase shifts can be introduced in one of the beams (object or reference) in one of the
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exposures of double exposure holography. In real-time applications, the phase shift can be
conveniently introduced in one of the beams during the reconstruction.
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Now we shall summarize the main features of the phase shifting procedure relevant to our
needs in two-color holographic interferometry.
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• General/review articles._'""8'u'z These articles describe theory and general methodology
of the technique. Besides the particular three step method described by eq. (2), several other
procedures are available. These are for example general three step method [more general
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form of what Equation (2) represents a particular case], four step method, the Carr6
method, the five step method, the integrating bucket method, multi-step method, etc.
Different phase measuring algorithms solve different purposes. For example, in the Carr6
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method, the phase shift need not be known. Four phase steps of equal (may be unknown)
amount are enough to evaluate qo.As we notice in the derivation of Equation (2), the phase
ii steps must be known.
• Articles dealing with errors.2"3° These are relatively recent works on the error sources,
their sensing, and possible ways to eliminate the effects on the measurements. These sources
are due to intensity variations during data collection, reference phase error, vibrations,
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nonlinearities of the photodetector, turbulence, etc. Iterative algorithms can be used to know
J correct reference phase values. _ Reference phase error can also be reduced by a
characteristic error function. Computer simulated _accuracy of an optical path then becomes
_./1000 ! A new algorithm and phase shifting via frequency translation z'helps in the phase
i error problem in the presence of vibration. This method is more relevant during testing
large optics. Creath _describes most common errors in phase-measuring interferometry and
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suitability of particular algorithms in specific error source situations. Van Wingerden et al. _
have performed an extensive study on these lines. In the method due to Lai and Yatagai _,
the reference phase is more correctly measured from parallel Fizeau fringes. Optical
surfaces with _./500 rms accuracy can then be measured. In the data averaging procedure
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due to Ovryn and Haacke, _the phase drifts can be compensated to obtain the 2/360 optical
path measurement accuracy by averaging 36 data sets. Ali and Wyant 3tconsidered the role
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of spurious reflections and then developed algorithms to eliminate the error.
• Articles dealing with fluids._'_gJ_6 Some works dealing with fluids using two reference
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beams are described in Section 2.2. Here we present some works with simple two beam
m interferometry (say real time holographic interferometry) and the digital technique. In fact,
that is what is done with NASA-FES holographic reconstructions. The work of Lanen,
Nebbeling and van Ingen 3'_ uses real-time holographic interferometry and digital phase-
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shifting to study a 2-D density around a heated horizontal cylindrical bar in free convection.
The phase steps in the reconstruction beam are introduced using a PZT (piezo-electric
transducer) which translates a mirror. Irradiances are then used to determine the whole field
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phase map and then the temperature field is computed. It is interesting to note that
although the digital procedure is well established, the application to fluid (gases) is relatively
new. Another similar procedure due to Dobbins et al. 3'measures temperature distribution
within a confined turbulent air jet impinging on a thermally conductive surface.
Some recent review articles clearly find phase measuring interferometry very useful to study
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transparent media. _'
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It is interesting to note that all these experimental test sections dealing with phase
measuring interferometry involve gases. An exception is Hariharan's _work where a fluid is
used indirectly in connection to two refractive index contouring of a surface. Thus, the use
W of phase measuring interferometry to liquids has been rare.
g 2.4 Effective technique for multi-color holography
It is evident that there are two techniques for very accurate fringe counting- heterodyne and
w phase shifting. Traditionally, the heterodyne approach has been the most sensitive. However,
the approach involves complex hardware and procedures. On the other hand, the phase
shifting procedure is relatively less sensitive but very practical. It is a whole-field procedure
u and there is minimum additional hardware (the phase shifter) needs. The conventional
sensitivity of the phase shifting procedure is more than what is required in multi-color
interferometry. Also, the sensitivity is rapidly approaching _° to the sensitivity obtainable by
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the heterodyne procedure. Therefore, it is logical to select the phase shifting procedure for
our present needs.
3 EXPERIMENTATION
At this stage, it is established that the phase shifting procedure is the most suitable approach
for our fringe counting needs in multi-color interferometry. The purpose of the current
experimentation is to establish our practical sensitivity in a typical NASA-FES environment.
The experiments were performed at the Space Science Laboratory at NASA/MSFC in
cooperation with NASA, MetroLaser, and UAH. The experiments provided valuable
wR experience, inputs for a design of a optical Two-Color Holographic Interferometry (T-CHI)
breadboard, and above all more than satisfied the sensitivity requirements of two- (or multi-
) color holography. Earlier works _'_found that if 1/100 of a fringe shift can be measured,
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then temperature and concentration in two-color interferometry can be separated. Our
w experimentation, using the existing hardware, established better than 1/200 'h fringe
measurement capability with room for further enhancement.
3.1 System description
t The sketch of the experimental arrangement is shown in Figure 1. Details of the hardware
(mostly existing at NASA/MSFC) are given in Appendix A. Radiations from HeNe (_. =
632.8 nm) and HeCd (£ = 441.6 nrn) were combined using a beam splitter. For the beam
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Granite table
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Mirror
[ HeNe laser [ ,
__ Beam splitter
[ HeCd laser ] :_
L (combiner)
Mirror [
// Reference _ _.-B]_atter
Detec.tor I/\/ /"/n _'- _'_,_'" ._. ] ]
i; =, \/- / u" :: _,,/x_./'t___../";,Mirrol r
i_ H°l°gram Test cell / I
k Phase shifter
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To oscilloscope
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Figure 1. Schematic diagram of the experimental set up for
two color holographic interferometry.
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combination, the cube type beam splitter was mounted on a translation-rotation stage. The
combined output was observed in the far field and the stage was adjusted for the overlap
so that the beam contained two colors. The rest of the alignment task is like usual
holography. The beam (containing two iaser outputs) are divided into two using a beam
splitter. These two beam spots are reflected and ultimately again superimposed at the center
of the hologram recording plane. The path lengths are adjusted to be equal for coherence
considerations. These adjustments are again refined later in the final system. Now, in each
beam path, spatial filters and collimators are introduced to obtain about 4- diameter cross-
section beams for the object and reference beams. In one of the paths (object in the present
set up) a rotating plate phase shifter is introduced. A test cell containing the fluids (sugar
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solution and/or water in our present experiments) is also introduced in the object beam
path. Real-time holographic interferometry was performed. For the purpose (see Figure 2),
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m Reference
_2 fRrienagl-etsime _ wave
stop ra / / wave
Aperture/ _ / Object
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_ shifter
D
Detector
_]_ _ _\L/ .,//.---__/,"/-'__@_Phase
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Test
_t_ H°l°gram cell
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Reconstructed and
real-time waves
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Figure 2. Observation and counting of real-time holographic fringe pattern for the analysis
of optical path variations of the fluid in the test cell.
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